Document new URL type.

Change-Id: I7d93fc398caa692b73e6f03ef3748bbc22c057d6
diff --git a/doc/indexing.md b/doc/indexing.md
index dc8bb1f..9bba9ec 100644
--- a/doc/indexing.md
+++ b/doc/indexing.md
@@ -8,4 +8,5 @@
 * `web-url`: base URL for linking to files, commits, and the repository, eg.
 `https://github.com/hanwen/usb`
 
-* `web-url-type`: type of URL, eg. github. Supported are cgit, gitiles, and cgit.
+* `web-url-type`: type of URL, eg. github. Supported are cgit,
+  gitiles, gitweb and cgit.